You don't have to be a mechanic to know when your wiper blades are going bad -- they'll tell you, in no uncertain terms. The message may come in the form of streaks on the windshield, or the wiper blades may "chatter" across the glass. Either way, this can contribute to a harrowing experience as the driver tries to peer through a windshield that has turned into an abstract painting instead of a clear vantage point.
Generally, wiper blades should be replaced at least every six months. Most are made of natural rubber, which can crack or harden through exposure to the sun and/or ozone. In heavily industrial areas, chemicals splashed up off the roadway can literally eat the rubber away over time. Road salt can also wreak havoc in wintertime, along with cold temperatures.
The newer wiper blades made of rubber compounds may last longer, but they are not immortal. The same things affect them, although it may take longer. Keep in mind that wiper blades are often scraping across abrasive sand and grit that has collected on the windshield.
Don't forget the rear wipers, either. Because they don't take the forward brunt of the aforementioned conditions, they may last longer, but they, too, will eventually fail. Rear wiper blades are also prone to damage from the brushes in automatic car washes.
Wiper blades are individualized to different types of automobiles. Part of the reason is that each vehicle has a slightly different cant to its windshield. Some windshields are dramatically curved to promote better aerodynamics, but that sharp angle can also allow wind to get under the wiper blades and lift them off the glass. Special wiper blades have been developed to negate this tendency.
As with any other automotive part, wiper blades run the gamut from stopgap to high performance. What works for you most likely depends upon how much you drive, and what part of the country you live in. It doesn't have to be raining to test your wiper blades -- simply squirt windshield cleaner onto the glass and see how the blades perform.
It is important to remember, though, that the wiper blades and the arms and holders to which they are attached function as a unit. If there is not enough spring tension in the arm, it may not hold the wiper blade firmly enough against the windshield. The blade itself may be in good shape, but is not being allowed to do its job.